Posterior-stabilized total knee prosthesis

ABSTRACT

In an orthopedic knee joint prosthesis, an intercondylar fossa of a femoral component cooperates with a spine formed in a tibial component to reproduce the screw home mechanism of a natural knee. When the femoral component and tibial component are positioned to correspond with slight flexion of the knee, the components are mutually rotationally locked against internal or external rotation. At higher degrees of flexion, such as greater than about 10-20 degrees of flexion, internal/external rotation of the tibia is permitted. The tibia is in an externally rotated position when locked, thereby reproducing the screw home mechanism and providing high stability.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under Title 35, U.S.C. § 119(e) ofU.S. Provisional Patent Application Ser. No. 61/146,745, entitledPOSTERIOR-STABILIZED TOTAL KNEE PROSTHESIS, filed on Jan. 23, 2009, theentire disclosure of which is expressly incorporated herein byreference.

BACKGROUND

1. Technical Field

The present invention relates to orthopedic prostheses and,specifically, to knee prostheses.

2. Description of the Related Art

Orthopedic prostheses are commonly utilized to repair and/or replacedamaged bone and tissue in the human body. For example, a kneeprosthesis may include a tibial component and/or a femoral componentthat replace damaged and/or destroyed bone in the tibia and/or femur andpromote articulation similar to the natural, anatomical articulation ofthe knee joint.

In a natural knee, internal rotation of the tibia occurs when the kneeis flexed from full extension (i.e., zero degrees flexion) to about 20degrees flexion, and, conversely, external rotation of the tibia occurswhen the knee is extended from about 20 degrees to full extension. Thisinternal/external rotation is known as the “screw home” mechanism. Thescrew home mechanism is driven in part by the difference in the radii ofcurvature of the medial and lateral condyles of the distal femur. Theexternally rotated orientation of the anatomic tibia in full extensionresults in tightening of cruciate ligaments and “locks” the knee againstinternal/external rotation at the tibia-femur interface. The screw homemechanism thereby promotes stability of the tibia with respect to thefemur when the knee is extended or slightly flexed.

In the final stages of knee extension, the tibia rolls anteriorly andthe posterior cruciate ligament (PCL) elongates, causing translation ofthe femur relative to the tibia at the tibia-femur interface. Thelateral side of the distal femoral articular surface experiences arelatively larger anterior translation as compared with the medial sideof same. This anterior movement of the lateral distal femoral articularsurface occurs during the last 20 degrees of knee extension results inexternal rotation of the tibia, and forms the basis for the screw homemechanism. Once in the extended position, internal/external rotation ofthe tibia is substantially prevented.

When the natural knee begins to flex from a position of fill extension,the lateral side of the distal femoral articular surface translatesposteriorly, elongating the anterior cruciate ligament (ACL). Thelateral femoral articular surface experiences a relatively largerposterior translation as compared with the medial side of same. Thisposterior movement of the lateral distal femoral articular surfaceoccurs during the first 20 degrees of knee flexion, and reverses thescrew home mechanism. Once the knee is sufficiently flexed, such asabout 20 degrees, internal/external rotation of the tibia is once againpermitted.

SUMMARY

The present disclosure provides an orthopaedic knee joint prosthesis inwhich an intercondylar fossa of a femoral component cooperates with aspine formed in a tibial component to reproduce the screw home mechanismof a natural knee. When the femoral component and tibial component arepositioned to correspond with slight flexion of the knee, the componentsare mutually rotationally locked against internal or external rotation.At higher degrees of flexion, such as greater than about 10-20 degreesof flexion, internal/external rotation of the tibia is permitted. As theknee joint prosthesis transitions from a flexion orientation to anextension orientation, the spine may interact with the intercondylarfossa to drive internal or external rotation of the knee, subsequentlylocking the tibia in the resulting rotated position.

In one embodiment, for example, a base portion of the tibial spine has awidth corresponding with a width between the inner faces of the lateraland medial condyles of the femoral component (i.e., the intercondylarfossa). A peak portion of the tibial spine has a reduced width which isless than the width of the intercondylar fossa. When the femoralcomponent and tibial spine are oriented in an extension orientation, theside walls of the intercondylar fossa closely engage the base of thespine to lock the tibia against internal/external rotation. When thetibial component and femoral component are in an extension orientation,such as at least 10 degrees of flexion, the side walls of theintercondylar fossa move so that they are adjacent the peak portion ofthe spine, thereby creating a space between the spine and the sidewallsof the intercondylar fossa that allows internal/external rotation.

In an alternative embodiment, the side walls of the intercondylar fossaof the femoral component may define a varying width along differentanteroposterior locations. For example, the intercondylar fossa maydefine a narrow width at an anterior position, which closely conforms tothe tibial spine to lock the tibia against external/internal rotationwhen the prosthesis is in an extension orientation. As the femoralcomponent articulates with the tibial component during flexion, theintercondylar fossa grows wider to provide a space between the sidewalls of the intercondylar fossa and the tibial spine, thereby creatinga space that permits internal/external rotation in larger amounts atlarger degrees of flexion.

In one form thereof, the present invention provides a knee jointprosthesis moveable between an extension orientation and a flexionorientation, the prosthesis including a femoral component and a tibialcomponent. The femoral component includes a lateral condyle having alateral condylar inner wall, a medial condyle having a medial condylarinner wall, and an intercondylar fossa bounded on two sides by thelateral condylar inner wall and the medial condylar inner wall. Thetibial component includes a tibial articulating surface, and a spineextending proximally from the tibial articulating surface. The spineincludes a base adjacent the tibial articulating surface, and the basehas a lateral base wall and an opposed medial base wall. A base width isdefined between the lateral base wall and medial base wall. The spineincludes a peak disposed proximally of the base, the peak having alateral peak surface and an opposed medial peak surface, with a peakwidth defined between the lateral peak surface and medial peak surface.The peak width is less than the base width. The lateral condylar innerwall cooperates with the lateral base wall, and the medial condylarinner wall cooperates with the medial base wall to prevent internalrotation and external rotation of the tibial component when the kneejoint prosthesis is in the extension orientation. The lateral condylarinner wall cooperates with the lateral peak surface, and the medialcondylar inner wall cooperates with the medial peak surface to permit atleast one of internal rotation and external rotation of the tibialcomponent when the knee joint prosthesis is in the flexion orientation.

In another form thereof, the present invention provides a knee jointprosthesis moveable between an extension orientation and a flexionorientation, the prosthesis including a femoral component and a tibialcomponent. The femoral component includes a lateral condyle having alateral condylar inner wall defining an anterior lateral wall segmentand a posterior lateral wall segment, a medial condyle having a medialcondylar inner wall defining an anterior medial wall segment and aposterior medial wall segment, and an intercondylar fossa bounded on twosides by the lateral condylar inner wall and the medial condylar innerwall. The intercondylar fossa includes an anterior space between theanterior lateral wall segment and the anterior medial wall segment, andthe intercondylar fossa includes a posterior space between the posteriorlateral wall segment and the posterior medial wall segment. The tibialcomponent includes an articulating surface and a spine extendingproximally from the tibial articulating surface, the spine having alateral spine wall and an opposed medial spine wall. The anterior spaceof the intercondylar fossa cooperates with the lateral spine wall andthe medial spine wall to prevent internal rotation and external rotationof the tibial component when the knee joint prosthesis is in theextension orientation. The posterior space of the intercondylar fossacooperates with at least one of the lateral spine wall and the medialspine wall to permit internal rotation and external rotation of thetibial component when the knee joint prosthesis is in the flexionorientation.

In yet another form thereof, the present invention provides a knee jointprosthesis moveable between an extension orientation and a flexionorientation, the prosthesis including a tibial component and a femoralcomponent. The tibial component includes tibial means for guidinginternal and external rotation of the tibial component, and the femoralcomponent includes a femoral means for guiding internal and externalrotation of the tibial component. The femoral means for guidingcooperates with the tibial means for guiding to prevent internalrotation and external rotation of the tibial component when the kneejoint prosthesis is in the extension orientation. The femoral means forguiding cooperates with the tibial means for guiding to (permit at leastone of internal rotation and external rotation of the tibial componentwhen the knee joint prosthesis is in the flexion orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features and advantages of thisdisclosure, and the manner of attaining them, will become more apparentand the invention itself will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, where:

FIG. 1A is an elevation, partial section view of a knee joint prosthesisin accordance with the present disclosure, shown in an extensionorientation;

FIG. 1B is an elevation, section, partial end view of the knee jointprosthesis shown in FIG. 1A, illustrating a multi-width spine;

FIG. 2A is an elevation, partial section, side view of the knee jointprosthesis shown in FIG. 1A, shown in a flexion orientation;

FIG. 2B is an elevation, section, end view of the knee joint prosthesisshown in FIG. 2A;

FIG. 3A is an plan view of the tibial component of the knee jointprosthesis shown in FIG. 1A;

FIG. 3B is an elevation, section view of the tibial component shown inFIG. 3A;

FIG. 4A is an elevation, section, plan view of a knee joint prosthesisin accordance with the present disclosure, shown in an extensionorientation;

FIG. 4B is a plan view of the knee joint prosthesis shown in FIG. 4A;

FIG. 5A is an elevation, section, end view of the knee joint prosthesisof FIG. 4A, shown in a flexion orientation;

FIG. 5B is a plan view of the knee joint prosthesis of FIG. 5A;

FIG. 6A is an elevation, section view of a femoral component of the kneejoint prosthesis of FIG. 4A; and

FIG. 6B is a plan view of the femoral component of FIG. 3B;

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the invention and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

As indicated above, the present disclosure provides a knee jointprosthesis which selectively allows or impedes internal/externalrotation of the tibia depending on the level of flexion of the knee.More particularly, the knee joint prosthesis of the present disclosurereproduces the screw home mechanism of a natural knee by preventinginternal/external rotation between full extension and a low level offlexion, i.e., less than about 10-20 degrees of flexion. The knee jointprosthesis permits internal/external rotation at higher levels offlexion. As the prosthesis is advanced from the low level of flexion tofill extension, the femoral component cooperates with the tibialcomponent to drive external rotation of the tibia. In order to preparethe tibia and femur for receipt of a knee joint prosthesis of thepresent disclosure, any known methods and apparatuses for preparation ofthe knee joint may be used.

Referring now to FIGS. 1A and 1B, knee joint prosthesis 10 includesfemoral component 12 and tibial component 14. Femoral component 12includes anterior end 16 and posterior end 18 (FIG. 1A), with a distalarticulating surface 20 extending therebetween. A proximal fixationsurface 22 is adapted for fixation to a resected distal femur using anyknown methods and apparatuses. Femoral component 12 includes lateralcondyle 24 and medial condyle 26, as best shown in FIGS. 1B and 2B.

Referring to FIGS. 1B and 2B, intercondylar fossa 28 is disposed betweenthe lateral condylar inner wall 30 of lateral condyle 24 and the medialcondylar inner wall 32 of medial condyle 26. At least a portion oflateral condylar inner wall 30 and medial condylar inner wall 32 definesa substantially flat or planar sagittal surface, i.e., the planesdefined by inner condylar walls 30, 32 are substantially parallel to asagittal plane. Thus, intercondylar fossa 28 defines a channel betweenlateral and medial condyles 24, 26 of femoral component 12. As describedin detail below, the channel-like nature of intercondylar fossa 28cooperates with spine 46 formed in tibial component 14 to reproduce thescrew-home mechanism of the natural knee.

As best seen in FIGS. 3A and 3B, tibial component 14 includes anteriorend 34 and posterior end 36, with articulating surface 38 extendingtherebetween. Fixation surface 40 (FIG. 3B) is adapted to attach to aresected proximal tibia by any known methods and apparatuses. Tibialcomponent 14 includes lateral compartment 42 and medial compartment 44,with lateral compartment 42 shaped and positioned to correspond withlateral condyle 24 of femoral component 12, and medial compartment 44shaped and positioned to correspond with medial condyle 26 of femoralcomponent 12. An intercondylar eminence, such as spine 46, is disposedbetween lateral and medial compartments 42, 44 and extends upwardly orproximally from articulating surface 38. Spine 46 cooperates withintercondylar fossa 28 of femoral component 12 (FIGS. 1B and 2B) tolimit internal/external rotation of tibial component 14 at certainlevels of flexion of knee joint prosthesis 10, and to permitinternal/external rotation at other levels of flexion, as describedbelow.

Referring to FIG. 2B, spine 46 includes base 48 and peak 50, with base48 disposed between tibial component body 39 and peak 50. Base 48defines lateral base wall 52 and medial base wall 54, each of which hasa substantially flat or planar sagittal surface, i.e., at least aportion of base walls 52, 54 define planar surfaces which aresubstantially parallel with a sagittal plane. Peak 50 defines lateralpeak surface 56 and medial peak surface 58 extending from the tops oflateral base wall 52 and medial base wall 54, respectively. Lateral andmedial peak surfaces 56, 58 taper to a summit 60 disposed at theproximal terminus of peak 50. Although the illustrated embodiment showspeak 50 as the uppermost (i.e., most proximal) portion of spine 46, withsummit 60 as the narrowest point of peak 50, it is within the scope ofthe present disclosure that peak 50 may also be disposed between base 48and another structure, such that summit 60 may abut a further proximalstructure forming a part of spine 46.

As best seen in FIG. 3B, medial base wall 54 is bounded by three edgesto form a generally triangular shape, with an anterior side of thetriangle coincident with the anterior face of peak 50 and two points ofthe triangle disposed on articular surface 38 of tibial component 14. Asillustrated in FIGS. 1A-2B and discussed in more detail below, this“triangular” shape allows base walls 52, 54 of spine 46 to cooperatewith inner condylar walls 30, 32 to selectively prevent or permitinternal/external rotation of tibial component 14 with respect tofemoral component 12 when knee joint prosthesis 10 toggled betweenextended and flexed orientations.

Referring now to FIGS. 2B and 3A, base 48 has width W_(B) definedbetween lateral base wall 52 and medial base wall 54. Width W_(B)corresponds with the width of intercondylar fossa 28. Lateral base wall52 is abutting or closely adjacent lateral condylar inner wall 30, andmedial base wall 54 is abutting or closely adjacent medial condylarinner wall 32 when tibial component 14 is in low-flexion or extensionorientations relative to femoral component 12. On the other hand, thetransverse distance between lateral peak surface 56 and medial peaksurface 58 is less than width W_(B), with such transverse distancetransitioning from being nearly equal to width W_(B) at the junctionbetween peak 50 and base 48, to being substantially less than widthW_(B) proximate summit 60 of peak 50. This reduced transverse width ofpeak 50 cooperates with intercondylar fossa 28 to allowinternal/external rotation of tibial component 14 relative to femoralcomponent 12 at certain flexion orientations of knee joint prosthesis10.

Width W_(B) of base 48 of spine 46 may be as little as 15 mm, 16 mm or17 mm, and as large as 20 mm, 23 mm, or 25 mm, or width W_(B) may bewithin any range delimited by any of the foregoing values. Similarly,the taper of peak 50, i.e., the reduction in the transverse width ofpeak 50 between base 48 and summit 60 may result in a width of summit 60that is as little as 10 mm, 12 mm or 14 mm, and as large as 16 mm, 18mm, or 20 mm, or the width of summit 60 may be within any rangedelimited by any of the foregoing values.

The clearance between condylar inner walls 30, 32 of intercondylar fossa28 and base 48 of spine 46 determines the extent of prevention ofinternal/external rotation in knee joint prosthesis 10, as described indetail below. This clearance may be as little as nearly zero mm, 0.03 mmor 0.06 mm, and as large as 0.10 mm, 0.15 mm, or 0.20 mm, or may bewithin any range delimited by any of the foregoing values.

Width W_(B), the taper of peak 50 and the clearance between base 48 andintercondylar fossa 28 may be chosen based on various designconsiderations, such as the overall size of knee joint prosthesis 10,the desired clearance between spine 46 and corresponding structures onfemoral component 12 (described below), and the like. For example, inone exemplary embodiment, width W_(B) of base 48 may be about 18.1 mmwide, with a near-zero clearance with intercondylar fossa 28. In thisembodiment, the transverse width of peak 50 may taper to about 15.0 mmat summit 60.

When knee joint prosthesis 10 is in an extension orientation, femoralcomponent 12 is positioned upon tibial component 14 such that a leg withknee joint prosthesis 10 implanted in the leg would be fully extended.In this extension orientation, illustrated in FIGS. 1A and 1B, at leasta portion of intercondylar fossa 28 is closely engaged with base 48.When so engaged, little or no gap exists between lateral condylar innerwall 30 of femoral component 12 and lateral base wall 52 of tibialcomponent 14. Likewise, on the medial side of knee joint prosthesis 10,little or no gap exists between medial condylar inner wall 32 of femoralcomponent 12 and medial base wall 54 of tibial component 14 in theextension orientation. As a result of the interaction between walls 30,52 and walls 32, 54, tibial component 14 and femoral component 12 arenot internally or externally rotatable relative to one another, i.e.,tibial component is fixed or locked against internal/external rotation.

Further, the interaction between walls 30, 52 and walls 32, 54 in theextension orientation defines the orientation of components 12, 14 withrespect to internal/external rotation. As will be described in moredetail below, this “locked” rotational orientation occurs after tibialcomponent 14 has been externally rotated in the final stages of flexion.This externally rotated orientation of tibial component 14 is similar toan anatomical knee which has externally rotated under the influence ofthe screw home mechanism as the knee is extended.

Referring from FIGS. 1A and 1B to FIGS. 2A and 2B, flexion of knee jointprosthesis 10 moves femoral component 12 relative to tibial component 14to a flexed orientation. In this flexed orientation, lateral and medialcondylar inner walls 30, 32 have moved posteriorly and proximally sothat walls 30, 32 are no longer engaged with lateral and medial basewalls 50, 54 of spine 46, respectively. Instead, condylar inner walls30, 32 are proximate lateral and medial peak surfaces 56, 58, of peak 50of spine 46. Because the transverse dimension of peak 50 is less thanwidth W_(B) of base 48, as discussed above, external or internalrotation of tibia 14 with respect to femoral component 12 becomespossible. Further, the gradual reduction of width of peak 50 from theinterface between peak 50 and base 48 to summit 60 of peak 50 results ina gradual increase in the ability of tibial component 14 to internallyor externally rotate with respect to femoral component 12. Thus, only asmall amount of internal/external rotation will be possible just afterintercondylar fossa 28 of femoral component 12 has disengaged base 48 oftibial component 14 (as shown in FIGS. 2A and 2B). As knee jointprosthesis 10 flexes further, however, intercondylar fossa 28 willengage the narrower proximal parts of peak 50, and moreinternal/external rotation will be permitted.

In the exemplary embodiment illustrated in FIGS. 1A-2B, intercondylarfossa 28 disengages from base 48 at between about 10 degrees to about 15degrees of leg extension. However, it is within the scope of the presentdisclosure that this disengagement may occur at as little as 5 degrees,10 degrees or 15 degrees and as much as 20 degrees, 25 degrees or 30degrees of extension, or within any range defined by any of theforegoing values. Complete disengagement of intercondylar fossa 28 fromspine 46 may or may not occur at a larger degree of flexion.

Referring from FIGS. 2A and 2B to FIGS. 1A and 1B, knee joint prosthesismay also be moved from a flexion orientation to an extensionorientation. When so moved, intercondylar fossa 28 of femoral component12 and spine 46 of tibial component 14 cooperate to urge externalrotation of the tibia in certain stages of flexion. In later stages offlexion and in extension, the rotational orientation of the tibiabecomes fixed in the externally rotated position, thereby replicatingthe screw home mechanism of an anatomic knee.

As tibial component 14 is articulated with femoral component 12 from ahighly flexed orientation toward extension, lateral and/or medial peaksurfaces 56, 58 of peak 50 cooperate with lateral and/or medial condylarinner walls 30, 32 to urge external rotation of tibial component 14 withrespect to femoral component 12. This urging becomes more pronounced asthe transverse width of peak 50 increases toward base 48 (as discussedabove). Thus, as intercondylar fossa 28 articulates with peak 50 ofspine 46 during extension, the tibia is smoothly externally rotatedtoward the external rotation orientation, and this externally rotatedorientation is locked throughout the final 10-20 degrees of flexion andin extension. This urged external rotation mimics the screw homemechanism of an anatomic knee, and may occur over a range of flexion,such as from about 45 degrees of flexion to between 20 and 10 degrees offlexion.

Referring now to FIG. 2A, as knee joint prosthesis 10 approaches thelow-flexion orientation, lateral and medial condylar inner walls 30, 32begin to reengage lateral and medial base walls 52, 54, respectively.This engagement prevents interior/exterior rotation of tibial component14 with respect to femoral component 12, thereby affecting the lockagainst further rotation. With intercondylar fossa 28 and base 48cooperating to lock tibial component 14 in an externally rotatedorientation, the tibia is placed in a high stability position that isconsistent with the screw home mechanism of an anatomic knee.

It is within the scope of the present disclosure that the urged externalrotation of tibial component 14 with respect to femoral component 12 mayoccur throughout any range of flexion, or may occur abruptly. Forexample, the peak portion of a tibial spine may feature an abrupttransition from the base to the peak, as opposed to the gradualtransition from base 48 to peak 50. This abrupt transition may take theform of a “step” or abrupt change in width, and results in the urgedexternal rotation of tibial component 14 occurring over a narrower rangeof flexion. Alternatively, the peak portion of the spine may be madetaller so that it extends further proximally. This taller peak mayinclude a gradually reducing transverse width, similar to peak 50 ofspine 46. The taller peak will allow the urged external rotation oftibial component 14 to occur over a larger range of extension.

Referring generally to FIGS. 4A-5B, knee joint prosthesis 110 includesfemoral component 112 and tibial component 114. Except where otherwisenoted, reference numbers of knee joint prosthesis 110 correspond withreference numbers of knee joint prosthesis 10, with reference numbers ofknee joint prosthesis 110 having 100 added thereto. Moreover, knee jointprosthesis 110 replicates the screw home mechanism of an anatomic knee,similar to knee joint prosthesis 10. However, intercondylar fossa 128 offemoral component 112 has varying widths corresponding to variousdegrees of flexion, while intercondylar eminence or spine 146 has asubstantially constant width, as described below.

As best seen in FIGS. 6A and 6B, femoral component 112 includes anteriorend 116 and posterior end 118 with articulating surface 120 extendingtherebetween. Fixation surface 122 disposed opposite articulatingsurface 120 is adapted for fixation of femoral component 112 to a distalresected femur. Referring to FIG. 6B, femoral component 112 includeslateral condyle 124 and medial condyle 126, with lateral condyle 124defining lateral condylar inner wall 130 and medial condyle 126 definingmedial condylar inner wall 132.

As best seen in FIGS. 4B, 5B and 6B, femoral component 112 includesintercondylar fossa 128, which is formed by the space between lateraland medial condylar inner walls 130, 132. Lateral condylar inner wall130 includes anterior lateral wall segment 130A and posterior lateralwall segment 130C, with transition wall segment 130B extendingtherebetween. Medial condylar inner wall 132 includes anterior medialwall segment 132A, posterior medial wall segment 132C and transitionalmedial wall segment 132B extending therebetween. Intercondylar fossa 128includes anterior space 128A disposed between anterior lateral andmedial wall segments 130A, 132A and defining width W_(A) (FIGS. 5B and6B), posterior space 128C disposed between posterior lateral and medialwall segments 130C, 132C and defining width W_(P) (FIGS. 5B and 6B).Transitional space 128B of intercondylar fossa 128 is disposed betweentransitional lateral and medial wall segments 130B, 132B and betweenanterior space 128A and posterior space 128C. As described in detailbelow, anterior transitional and posterior spaces 128A, 128B, 128Ccooperate with tibial component 114 to permit or preventinterior/exterior rotation of tibial component 114 with respect tofemoral component 112, depending on the flexion orientation of kneejoint prosthesis 110.

Referring now to FIGS. 4A-5B, tibial component 114 defines tibialcomponent body 139 having anterior end 134 and posterior end 136, witharticulating surface 138 extending therebetween. A fixation surface 140(FIGS. 4A and 5A) is disposed opposite articulating surface 138, and isadapted to attach to a resected proximal tibia by any known methods andapparatuses. Articulating surface 138 includes lateral compartment 142and medial compartment 144, with intercondylar eminence or spine 146disposed therebetween and extending upwardly or proximally fromarticulating surface 138. Spine 146 includes lateral spine wall 152 andmedial spine wall 154, with spine walls 152, 154 disposed mutuallyopposite one another in a generally parallel configuration. Spine walls152, 154 are shown as having a generally planar configuration, but mayalso be rounded.

Referring now to FIGS. 4A and 4B, knee joint prosthesis 110 is shown inan extension orientation in which anterior lateral wall segment 130Aabuts or is closely adjacent lateral spine wall 152, and anterior medialwall segment 132A abuts or is closely adjacent medial spine wall 154.Therefore, anterior space 128 a of intercondylar fossa 128 capturesspine 146, thereby preventing internal/external rotation of tibialcomponent 114 with respect to femoral component 112.

Referring now to FIGS. 5A and 5B, when knee joint prosthesis 110 movesfrom extension to low flexion, such as about 10 to 20 degrees offlexion, spine 146 moves out of anterior space 128A of intercondylarfossa 128 and into transitional space 128B. Therefore, lateral spinewall 152 is proximate transitional lateral wall segment 130B, and medialspine wall 154 is proximate transitional medial wall segment 132B. Someinternal/external rotation of tibial component 114 with respect tofemoral component 112 will now be permitted.

As knee joint prosthesis 110 is flexed further, such as to up to about45 degrees, spine 146 exits transitional space 128B and enters posteriorspace 128C of intercondylar fossa 128. In this orientation, lateralspine wall 152 is proximate posterior lateral wall segment 130C andmedial spine wall 154 is proximate posterior wall segment 132C, andconstraint on internal/external rotation of tibial component 114 withrespect to femoral component 112 is further relaxed.

When knee joint prosthesis 110 is articulated from a flexed orientationback to an extension orientation, spine 146 moves from posterior space128C into transitional space 128B and eventually into anterior space128A of intercondylar fossa 128. Similar to the gradually changing widthof peak 50 of spine 46 (discussed above), the gradual reduction fromwidth W_(P) of posterior space 128C to the smaller width W_(A) ofanterior space 128A and transitional space 128B helps guide spine 146into the locked position corresponding with an extension or low flexionorientation of knee joint prosthesis 110. Further, this gradualtransition occurring in transitional space 128B urges tibial component114 to externally rotate, so that tibial component is in an externallyrotated extended position when locked against further rotation. Asdiscussed above, this locked, externally rotated position promotesstability of knee joint prosthesis 110. Moreover, knee joint prosthesis110 mimics or reproduces the screw home mechanism of an anatomic kneejoint.

Widths W_(A), may cooperate with the width of spine 146 to providevarying levels of clearance between spine 146 and intercondylar fossa128, as described above with respect to knee joint prosthesis 10.Moreover, in certain embodiments, the width of spine 146 may generallycorrespond with width W_(B) of base 48 of spine 46, while the differencebetween widths W_(A) and W_(P) of intercondylar fossa 128 may generallycorrespond with the difference between difference between width W_(B) ofbase 48 and the width of summit 60 resulting from the tapering of spine46.

Advantageously, knee joint prostheses 10, 110 promote stability of aknee joint by preventing the potentially destabilizing influence ofinternal or external rotation during the last stages of knee extension,i.e., the last 10 to 15 degrees of extension. Prior to the terminalextension phase, external rotation of the tibia is urged by knee jointprosthesis 10, 110 to orient the tibia in a highly stabile position, andto lock the tibia against internal/external rotation in that position.This urged external rotation and subsequent locking action is similar tothe screw home mechanism of an anatomic knee, and therefore facilitatesbehavior of knee joint prostheses 10, 110 that more closely approximatesa healthy anatomic knee joint.

The illustrated embodiments herein illustrate knee prostheses 10, 110adapted for use in a right knee. However, the principles of the presentdisclosure are also applicable to applications in a left knee.

While this invention has been described as having exemplary designs, thepresent disclosure can be further modified within the spirit and scopeof this disclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the disclosure using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallswithin the limits of the appended claims.

What is claimed is:
 1. A knee joint prosthesis moveable between anextension orientation and a flexion orientation, the prosthesiscomprising: a femoral component comprising: a lateral condyle having alateral condylar inner wall defining an anterior lateral wall segmentand a posterior lateral wall segment; a medial condyle having a medialcondylar inner wall defining an anterior medial wall segment and aposterior medial wall segment; and an intercondylar fossa bounded on twosides by said lateral condylar inner wall and said medial condylar innerwall, said intercondylar fossa including an anterior space between saidanterior lateral wall segment and said anterior medial wall segment,said intercondylar fossa including a posterior space between saidposterior lateral wall segment and said posterior medial wall segment;and a tibial component comprising: a tibial articulating surface; aspine extending proximally from said tibial articulating surface, saidspine having a lateral spine wall and an opposed medial spine wall; saidanterior space of said intercondylar fossa cooperating with said lateralspine wall and said medial spine wall to prevent internal rotation andexternal rotation of said tibial component when the knee jointprosthesis is in the extension orientation; and said posterior space ofsaid intercondylar fossa cooperating with at least one of said lateralspine wall and said medial spine wall to permit internal rotation andexternal rotation of said tibial component when the knee jointprosthesis is in the flexion orientation; wherein said spine of saidtibial component first disengages said inner walls of said intercondylarfossa of said femoral component from an engaged position at anorientation of the knee prosthesis corresponding to about 10 degreesflexion to about 45 degrees flexion.
 2. The knee joint prosthesis ofclaim 1, wherein said spine defines a spine width between said lateralspine wall and said medial spine wall, said anterior space of saidintercondylar fossa defines an anterior width corresponding to saidspine width, and said posterior space of said intercondylar fossadefines a posterior width larger than said anterior width.
 3. The kneejoint prosthesis of claim 1, wherein: said lateral condylar inner wallof said lateral condyle further defines a transitional lateral wallsegment disposed between said anterior lateral wall segment and saidposterior lateral wall segment; said medial condylar inner wall of saidmedial condyle further defines a transitional medial wall segmentdisposed between said anterior medial wall segment and said posteriormedial wall segment; and said intercondylar fossa further including atransitional space between said transitional lateral wall segment andsaid transitional medial wall segment.
 4. The knee joint prosthesis ofclaim 3, wherein said spine defines a spine width between said lateralspine wall and said medial spine wall, said anterior space of saidintercondylar fossa defines an anterior width corresponding to saidspine width, said posterior space of said intercondylar fossa defines aposterior width larger than said anterior width, and said transitionalspace of said intercondylar fossa defines a reduction from saidposterior width to said anterior width over a plurality of intermediatewidths that each are wider than the anterior width and narrower than theposterior width.
 5. The knee joint prosthesis of claim 1, wherein theflexion orientation of the knee prosthesis begins when said spine firstdisengages said anterior lateral wall segment and said anterior medialwall segment of said intercondylar fossa at an orientation of 20 degreesflexion.
 6. A knee joint prosthesis moveable between an extensionorientation and a flexion orientation, the prosthesis comprising: afemoral component comprising: a lateral condyle having a lateralcondylar inner wall; a medial condyle having a medial condylar innerwall; an anterior portion connecting said lateral condyle and saidmedial condyle; and an intercondylar fossa bounded on two sides by saidlateral condylar inner wall and said medial condylar inner wall, saidintercondylar fossa having varying widths between said lateral condylarinner wall and said medial condylar inner wall; wherein said varyingwidths of said intercondylar fossa includes: a first width extendingalong an anterior portion of the intercondylar fossa that is nearlyequal to a width of the spine, said anterior portion includes a firstset of parallel segments of said lateral condylar inner wall and saidmedial condylar inner wall; and a second width extending along aposterior portion of the intercondylar fossa greater than the firstwidth, said posterior portion includes a second set of parallel segmentsof said lateral condylar inner wall and said medial condylar inner wall;a transition portion formed by non-parallel segments of said lateralcondylar inner wall and said medial condylar inner wall; and a tibialcomponent comprising: a tibial articulating surface; and a spineextending proximally from said tibial articulating surface, said spineincluding a lateral spine wall and an opposed medial spine wall; andwherein said lateral condylar inner wall configured to engage with saidlateral spine wall and said medial condylar inner wall configured toengage with said medial spine wall, to prevent internal rotation andexternal rotation of said tibial component when the knee jointprosthesis is in the extension orientation; and said lateral condylarinner wall configured to disengage with said lateral spine wall and saidmedial condylar inner wall configured to disengage with said medialspine wall, to permit at least one of internal rotation and externalrotation of said tibial component when the knee joint prosthesis is inthe flexion orientation.
 7. The knee joint prosthesis of claim 6,wherein a difference between a first width between said first set ofparallel segments and a second width between said second set of parallelsegments of the intercondylar fossa corresponds to a difference betweena third width of a base of the spine and a fourth width of a summit ofthe spine, which results in a tapering of the spine.
 8. The knee jointprosthesis of claim 6, wherein said transition portion is locatedbetween and connects said first width and said second width.